Missing biding to Foreground in the ContentPresenter of the control which prevents not styled controls (i.e. TextBlock) not to receive the proper Foreground for the current theme.

Workaround:

add Foreground = "{TemplateBinding Foreground}" to the ContentControl in the ControlTemplate for RadBusyIndicator.

Scheduled for: 
The fix for this issue will be available with LIB (version 2018.3.1203) published on Monday, 3-rd December, 2018.